Certain types of gas discharge lamps, such as one class of fluorescent lamps, include a pair of cathodes each of which incorporates an internal resistance that becomes heated when a respective current passes therethrough; such cathodes are referred to hereinafter as resistively heated cathodes. The "resistive" heating occurs both during steady state lamp operation, when the cathodes are also heated from an arc discharge in the lamp, and during a so-called cathode pre-heat period, prior to lamp turn-on. The cathodes of such lamps are designed to emit electrons during normal lamp operation. Such cathodes typically comprise tungsten or similar metal, which, when uncoated, is susceptible to fracturing when heated to emit electrons. The cathodes, therefore, are typically coated with an electron-emissive material, to facilitate electron emission, while protecting the cathode metal from fracturing.
It is desirable, in a fluorescent lamp of the mentioned type, for the cathodes to be heated to at least about 700.degree. Centigrade (C) during the cathode pre-heat period, prior to lamp turn-on, to achieve a desired themionic emission of electrons from the cathodes. During steady state lamp operation, a continued heating of the cathodes, to about 500.degree. C. is desirable to maintain a preferably, thermionic emission of electrons from the cathodes and long cathode life.
A typical power supply, or (as typically described) "ballast," circuit for a cathode-heated type of fluorescent gas discharge lamp utilizes a positive temperature coefficient (PTC) resistor in a circuit for heating the lamp cathodes, both during the cathode pre-heat period, and during steady state lamp operation. The gas discharge lamp has a pair of resistively heated cathodes, each of which has a respective terminal coupled to a resonant power supply circuit for supplying bidirectional current to the lamp. Across the other pair of cathode terminals, a positive temperature coefficient (PTC) resistor is respectively coupled, via a serially connected capacitor, to complete a circuit for supplying current to, and hence heating, the resistively heated cathodes. Examples of ballast circuits utilizing PTC resistors can be found in U.S. Pat. Nos. 4,647,817; 4,782,268 and 5,122,712.
The PTC resistor initially conducts current at one impedance level, and increases in impedance level as it becomes heated through dissipating energy. Thus, when the lamp ballast circuit is initially energized, a relatively high current flows through the PTC resistor, and hence through the resistively heated cathodes. Such rapid heating occurs during a cathode pre-heat period, before lamp turn-on, to achieve a desirably high temperature of the lamp cathodes for initiating lamp turn-on. The PTC resistor is chosen so that it transitions to a high impedance state near the end of the cathode pre-heat period, when it allows the lamp voltage to increase to a point sufficient to initiate lamp turn-on. Thereafter, during steady state lamp operation, the lamp voltage falls to a substantially lower level than during the cathode pre-heat period. During this steady state lamp operation, a reduced current flows through the resistively heated cathodes, resulting in less power dissipation in the PTC resistor of, for instance, on the order of 1 watt for a 20-watt lamp ballast circuit, representing a considerable energy inefficiency, of about 5%.
It would, therefore, be desirable to provide a ballast circuit for a cathode-heated type of gas discharge lamp that realizes a higher power efficiency during steady state lamp operation.
A further drawback of the above-described ballast circuit is the limited range of resistively heated cathodes for which a given PTC resistor is applicable. Such resistively heated cathodes are required in a variety of types (e.g. 2-ohm, 6-ohm, etc.), to accommodate different types of lamps. Cathode heating circuitry that is more adaptable to different types of resistively heated cathodes would thus be desirable, to more easily accommodate a greater variety of lamp types.
Moreover, it would be desirable to provide the foregoing advantages without the addition of expensive or of bulky circuitry. The avoidance of bulky circuitry is especially important for a class of compact, low pressure fluorescent lamps that employ a standard Edison-type screw base, for installation in a conventional lamp socket also accommodating incandescent lamps, and that employ a compact, multi-axis envelope, or discharge vessel, in which light is emitted from a suitable fill that is electrically excited to a discharge state. The ballast circuit for such compact fluorescent lamp is compactly contained in, and immediately adjacent, the Edison-type screw base, and is thus under rigid size constraints.